The rounding can be a circle or an ellipse, or if one of the value is 0 no rounding is done and the corner is square.

A background, being an image or a color, is clipped at the border, even a rounded one; the exact location of the clipping is defined by the value of the background-clip property.

If the value of this property is not set in a border-radius shorthand property that is applied to the element after the border-bottom-right-radius CSS property, the value of this property is then reset to its initial value by the shorthand property.

all elements; but User Agents are not required to apply to table and inline-table elements when border-collapse is collapse. The behavior on internal table elements is undefined for the moment.. It also applies to ::first-letter.

Syntax

the value is a <length> or a <percentage> denoting the radius of the circle to use for the border in that corner.

With two values:

the first value is a <length> or a <percentage> denoting the horizontal semi-major axis of the ellipsis to use for the border in that corner.

the second value is a <percentage> denoting the vertical semi-major axis of the ellipsis to use for the border in that corner.

Values

<length-percentage>

Denotes the size of the circle radius or the semi-major and semi-minor axes of the ellipsis. As absolute length it can be expressed in any unit allowed by the CSS <length> data type. Percentages for the horizontal axis refer to the width of the box, percentages for the vertical axis refer to the height of the box. Negative values are invalid.

[1] The prefixed version (allowed from Firefox 1 to Firefox 12) of this property uses a different name, -moz-border-radius-bottomright.

[2] Before Firefox 4, the <percentage> was relative to the width of the box even when specifying the radius for a height). This implied that -moz-border-radius-bottomright was always drawing an arc of circle, and never an ellipse, when followed by a single value.

[3] Prior to Gecko 50.0 (Firefox 50.0 / Thunderbird 50.0 / SeaMonkey 2.47), border styles of rounded corners were always rendered as if border-style was solid. This has been fixed in Gecko 50.0.